firmware: fw_base.S: Fix boot hart status synchronization

It's wrong to put the fence after setting the boot status flag because
all relocation operations must be finished before setting the status
flag. So, this fence must be put before the setting status flag, and
there is no use in putting a fence between _start_warm and setting
status flag.

Also, nop can't delay other harts too much, so use div instead, just
like Linux cpu_relax. Current opensbi force enables “M” Standard
Extension, and mul instructions have been used in the fw_base.S.

After the above two fixes, the boot hart index param of the
fw_dynamic_info could be guaranteed properly for all platforms.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>